This invention relates to monitoring a pellicle to determine when it wears out. More particularly it relates to methods and structures to monitor pellicles subject to exposure wavelengths that can cause pellicle thinning.
Typically people look at the pellicle in monochromatic light and look for signs that the mask pattern is xe2x80x9cprintingxe2x80x9d onto the pellicle surface. The technique is not quantitative and not very sensitive. It also is sensitive to the wrong thing, namely thin-film interference within the pellicle, not externally measured change in optical path length. Thin film interference is sensitive to changes in nt (where n is the refractive index and t is the thickness), while external optical path length is sensitive to changes in (nxe2x88x921)t.
An example of another system that is intended to determine pellicle life is provided by Japanese Application Number JP19870111206 19870506 entitled xe2x80x9cExposure Devicexe2x80x9d by Mitsubishi Electric Corporation. In that published application a light flux from an optical system is focused on a photo detector for measuring light intensity at wafer level. The light intensity is measured with and without a pellicle to determine the transmissivity of the pellicle. When the transmissivity of the pellicle falls below a certain set value the pellicle is considered worn out. Again this is another way of determining wear out by transmission loss.
Pellicles can be degraded by the exposure light during use. For 365 and 248 nm, this has not been a major problem, because the pellicle materials in use are very resistant to damage at those wavelengths. At increasingly short wavelengths, such as 193 and 157 nm, pellicles are much more easily damaged. Damage can show up as a change in transmission, thickness, index of refraction, or a combination of all three. Changes in pellicle transmission lead to dose changes during wafer exposure. This will cause image size changes which can readily be detected.
However, non-uniform changes in optical path length of the pellicle (a function of thickness and index of refraction) can cause image position displacements which are not easily detected. It has proven extremely difficult to quantitatively measure the amount of optical path length change induced by photo-induced pellicle damage. This means that there is a risk of using a pellicle that has begun to induce optical distortions to the mask beneath it, or conversely to discard a pellicle out of excessive caution, before its life is over. This invention teaches a mask structure and method of quantitatively measuring pellicle degradation in production photomasks, by measuring overlay in test structures on the mask. A set of test structures that can be measured are placed in a transition region where the relative movement between them provides an indication of degradation. Once a predetermined displacement has taken place that impacts the quality or yield of the lithography process, the pellicle is considered worn out. Reference structures and additional monitor structures can be added to the mask to correct for displacement due to stepper error. Even more particularly a mask which comprises a first and second test structure; one structure located in a high transmission region close to a transition boundary between high and low transmission regions of the mask such that pellicle degradation impacts the printing of the object; the second structure located in the low transmission region of the mask close to the transition boundary such placement such that the structures will overlay each other. A second set of structures is used for control purposes to see if the difference in overlay is due to other factors The method then compares these structures to determine the image displacement due to degradation that is taking place.